[5] In 1900, 1902, 1904, and 1906, he refused requests to run again for Congress in the 8th district, concluding each time that incumbent Republican Congressman William P. Hepburn could not be defeated.

In 1906, Porter ran for governor of Iowa, winning the Democratic nomination,[8] but losing to incumbent Republican Albert B. Cummins.

[9] The Iowa General Assembly, which retained the power to choose U.S. senators from among the parties' nominees, twice selected Cummins over Porter, in a November 1908 vote (resolving who would serve the rest of Allison's original March 1903 – 1909 term) and a January 1909 vote (resolving who would serve the March 1909 – 1915 term).

Senator in 1911, when the entire contest was decided in the Iowa General Assembly without a primary due in part to Dolliver's death.

After Democrat Woodrow Wilson was elected president, Porter was nominated and confirmed as U.S. Attorney for the Southern District of Iowa, where he served from 1914 to 1918.

While serving as U.S. Attorney, he aggressively enforced the Espionage Act of 1917 against persons who spoke out against the draft or "assisted" others who did so, including the defendants in the 1917–18 Davenport sedition trial.

[14] He also served as first assistant special prosecutor in the Chicago trial of over one hundred members of the Industrial Workers of the World on similar charges.

[15] After a trial that lasted from April to August 1918, the jury deliberated briefly and returned convictions of all 100 remaining defendants, including IWW general secretary Big Bill Haywood.

Porter's chances for a victory in a statewide election were the greatest in 1926, when he again ran for the U.S. Senate, this time against insurgent Republican Smith W. Brookhart.